I know about the issue of one side of the sine wave being compressed, have seen it myself but maily on screen grid tubes. If you have a screen grid tube like a 6GH8 as a preamp, you can adjust the G2 voltage to find the most linear spot. I do it by driving the stage to near clipping with a triangular wave, and adjusting the screen voltage until the sides of the triangle are flat. This is applied mostly to power amps I have reworked which use a 6GH8 pentode for the input amp and the triode for the phase splitter. I like the sound of a tube power amp very much.
